Submitted for the 4CF15 Meeting of The American Physical SocietyElectron affinity of cubic boron nitride terminated with vanadium-oxide YU YANG, TIANYIN SUN, JOSEPH SHAMMAS, MAN-PUNEET KAUR, MEI HAO, ROBERT NEMANICH, Arizona State University -H-terminated cubic boron nitride (c-BN) has been shown to exhibit a negative electron affinity (NEA) surface, which may enable applications in thermionic and photon-enhanced energy conversion devices. The ability to withstand high temperature operation is an important factor in the thermionic emission applications. Theoretical and experimental studies have indicated that transition metal oxides can significantly influence the electronic properties of diamond. In this study, a thermally stable NEA for a c-BN surface with vanadium-oxide-termination is achieved, and its electronic structure was analyzed with in-situ photoelectron spectroscopy. Thin vanadium layers of˜0.1 and 0.5 nm were deposited on the c-BN surface in an electron beam deposition system. Oxidation of the metal layer was achieved by an oxygen plasma treatment. After 650 o C thermal annealing, the vanadium oxide on the c-BN surface was determined to be VO 2 , and the surfaces were found to be thermally stable, exhibiting an NEA. In comparison, the oxygen-terminated c-BN surface, where B 2 O 3 was detected, showed a positive electron affinity (PEA) of˜1.2 eV. The B 2 O 3 evidently acts as a negatively charged layer introducing a surface dipole directed into the c-BN. Through the interaction of VO 2 with the B 2 O 3 layer, a B-O-V layer structure would contribute a dipole between the O and V layers with the positive side facing vacuum. The lower enthalpy of formation for B 2 O 3 is favorable for the formation of the B-O-V layer structure, which provides a thermally stable surface dipole and a NEA surface.

No abstract

Cubic boron nitride (c-BN) has been deposited on nitrogen-doped polycrystalline diamond films via plasma-enhanced chemical vapor deposition employing fluorine chemistry. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were utilized to characterize the c-BN/diamond interface. TEM observations indicated local epitaxy of c-BN on diamond, while h-BN was also observed at the interface. XPS measurements indicated that c-BN growth continued after nucleation. The band offsets between c-BN and diamond were deduced from XPS measurements. The c-BN valence band maximum (VBM) was 0.8 ± 0.1 eV above the diamond VBM, which corresponded to the c-BN conduction band minimum (CBM) of 1.7 ± 0.1 eV above the diamond CBM. Comparison with offsets predicted by theoretical calculations suggests that a C-N interface was obtained.